An analysis of the existing metal drawing techniques indicates that the amount of energy required to overcome the contact friction forces in the deformation zone increases with an increase in the specific surface of the blank, i.e. with a reduction in the diameter.
As a result, the degree of deformation effected in one pass is very small, which accounts for a low productivity and makes the drawing process totally inapplicable to the production of small-diameter microwire. There are also considerable difficulties involved in the fabrication of microwire drawing equipment.
The use of active lubricants, the polishing of the working surfaces of drawing dies and other measures of this kind do not significantly reduce contact friction forces.
Thus contact friction forces continue to be a limiting factor in microwire drawing processes.
Unlike the above-mentioned measures, contact friction forces can be significantly reduced by drastically reducing the area of contact between the die and the blank. This means changing the traditional drawing techniques.
Today there are two basic methods for the production of microwire less than 20 mu in diameter:
pulling a microwire from a melt in a glass or ceramic sheath; PA1 drawing a microwire through hard-alloy, diamond and other equivalent dies.
When drawn in a sheath from a melt or an alloy, a microwire suffers from non-uniform cross-sectional dimensions. Such wire is further disadvantageous in that it has a cast structure and displays inferior mechanical properties. Finally, provision has to be made for chemical removal of the sheath.
It is very hard to manufacture a microwire less than 20 mu in diameter by drawing it through hard-alloy or diamond dies of conventional designs, bearing in mind the great specific surface, great amount of energy required to overcome friction forces, complexities involved in the die fabrication, and small deformation of the blank per pass.
A decrease in the diameter affects the mechanical strength of microwire; on the other hand, the specific surface and forces of friction increase. The factor which determines the minimum possible diameter of a microwire is the stretching strain.
The above problem can be solved only partially through the use of active lubricants, ultrasound, etc.
There is known a method for microwire manufacture, which consists of drawing a microwire blank through a die (cf. U.S. Pat. No. 3,955,390).
According to this method, twisted microwire is pulled through a die at whose outlet there are arranged rollers which discontinue the plastic twisting of the wire and thus prevent excessive plastic deformation.
Although twisting of the wire prior to the drawing process increases the yield strength of the wire in the course of twisting and the degree of deformation per pass, it provides only a partial solution to the problem of efficiency, bearing in mind that drawing metal through a die continues to be a complicated and time-consuming process.
The foregoing wire drawing method is carried out with the aid of a device (cf. FRG Application No. 2,449,474, Cl. B. 21C 3/06, of Jan. 15, 1976) comprising a body and a die composed of several parts arranged in the body and forming a drawhole through which a wire blank is pulled. The device also contains a wire drawing mechanism.
In the course of the drawing process, the wire blank interacts with the above-mentioned parts of the die, whereof each has an inlet surface, a working surface and an outlet surface.
The device further includes a number of adjustment means to control the spacing between individual parts of the die. As a wire blank is drawn through the die, it comes in contact with the entire surface of the deformation zone. Clearly, the device under review displays all the drawbacks inherent in dies in general. Much energy is required to overcome friction forces. Wire breakages are quite frequent. A small reduction per pass makes wire drawing an arduous and time-consuming process. Finally, a number of serious difficulties are involved in the die manufacture.